Process monitoring and control systems, such as those used in chemical processes, petroleum, or other types of industrial processes, typically include a centralized monitoring and control system communicatively coupled to a workstation of an operator or user and to one or more field devices via analog or digital communication paths. Field devices can be sensors adapted to monitor process parameters (such as temperature, pressure, flow rate, and the like) and/or transducers adapted to perform operations on the industrial process (such as opening and closing valves, and so on).
Generally, the centralized monitoring and control system receives signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices via an input/output (I/O) device or module, which may be analog or digital. Monitoring systems receive signals indicative of process measurements and monitor various aspects of a process based on the received signals. Monitoring systems can be adapted to compare measured process data against pre-determined limits and to initiate an action (such as generating an alarm signal) if the limit is exceeded.
A process controller of a monitoring and control system can use the measurements and other information to monitor a process and to implement a control routine. The process controller can generate control signals, which can be sent over buses or other communication paths or channels via an analog or digital I/O device to the field devices to control the operation of a particular process.
Conventionally, various communication protocols were developed to enable controllers and field devices from different manufacturers to exchange data. One such communication protocol is a MODBUS protocol, for example. The MODBUS protocol defines a message structure that controllers can recognize and use regardless of the types of networks over which they communicate. The MODBUS protocol is well known in the art and is described in detail in numerous articles, brochures, and specifications that are readily available from various sources including manufacturers of devices that utilize the MODBUS protocol. Other communication protocols include, for example, HART®, PROFIBUS®, actuator sensor interface (“AS-Interface”), WORLDFIP®, Device-Net®, CAN, and FOUNDATIONTM FIELDBUS™ (hereinafter “fieldbus”) protocols.
Regardless of the communications protocol used, power delivery to individual field devices and to interface modules and junctions within process monitoring and control networks impose costs. In particular, cabling itself and the cost of cable installation increase the overall costs of such systems. Moreover, when adding new devices to network, stringing additional cables to deliver power and signals to the new devices adds to the costs, complexity, and installation time.
Recently, a new standard has emerged (IEEE 802.3af) relating to power delivery to distributed systems. Specifically, the standard involves delivering power over existing Ethernet cabling utilizing unused pairs (or signaling pairs) of wires within the cabling. This delivery of electrical power over Ethernet cables is referred to as “Power over Ethernet” (PoE). The IEEE standard allows for 48-volts and 350 mA to be delivered over the same Ethernet cabling (typically CAT5E cabling) as the Ethernet communications.
Therefore, there is ongoing need for industrial process devices that can take advantage of emerging power delivery techniques and existing cabling for coupling new field devices to process monitoring and control systems.